Function render_key 

pub fn render_key(tree: &PatchTree, spec: &PhraseSpec) -> String

Content address of one (term, spec) featurization, 32 lowercase hex chars.

The spec is folded in because φ is only defined relative to the stimulus: two engines with different phrases must never share an entry. A 0xff separator (not a valid byte anywhere in either JSON) keeps the concatenation unambiguous.